The present invention pertains to electrical measurements in general and in particular, to a broadband current detector and an instrument for measuring impedance and apparatus for measuring power that uses the same.
Current measurement is a basic measurement and measurement of current flowing through floating lines is widely used. There are also many cases where, besides simply measuring current, current is measured as one of several measurements of physical and chemical quantities other than electricity including measurements of quantities related to power and impedance.
The method whereby the current to be measured is introduced through a balanced-to-unbalanced transformer (referred to below as a balun), such as transformer coupling, etc., to a current detector or voltage detector, which are unbalanced apparatuses, has been used for resultant determination of current with said current detector or voltage detector. However, ideal transformer coupling cannot be used for determinations of current within a wide frequency range of, for instance, 1 MHz to 1 GHz, so that a transmission path-type balun with relatively good frequency properties is used.
A transmission path-type balun is made, for instance, by coiling a coaxial line around a ferrite core, with one terminal pair of said coaxial line serving as the pair of input terminals and the other terminal pair serving as the pair of output terminals. The coupling coefficient of the center conductor and the sheath of the coaxial line are very close to 1 and therefore, excellent frequency properties are achieved. There is a reduction in applied voltage to the component to be measured due to self-inductance of the balun and therefore, as a means for preventing this, the coaxial line is coiled around the ferrite core in order to increase self-impedance and alleviate said reduction. For instance, an example of the use of a transmission path-type balun is given in Japanese Kokai Patent No. 9[1997]-318671.
FIG. 1 is a simplified circuit diagram of an instrument for measuring impedance that is a preferred example of using a current detector that uses this type of transmission path-type balun. Complex impedance Zx of the component to be measured 18 is determined as the vector ratio (v1/i1) of current i1 flowing through said component 18 and voltage v2 applied across said component 18. Incidentally, the current flowing through capacitor 34 and the current flowing through another parasitic impedance will have an effect on i1, but these currents are disregarded in the description of the present invention to simplify the description. Direct-current power source 12 and alternating current power source 10, power source resistance 14, direct-current detection resistance 16 and component to be measured 18 are connected in-series. Current detection resistance 16 is such that the pair of input terminals of transmission path-type balun 20 represents the end. One pair of the output terminals of balun 20 is direct-current coupled with the terminal on the power source resistance 14 side of current detection resistance 16 via balun 20 and coupled to reference potential point 4 (often has ground potential) via capacitor 24. The other pair of output terminals of balun 20 is direct-current coupled with the terminal on side of current detection resistance 16 of the component to be measured 18 via balun 20 and coupled to reference potential point 4 via capacitor 30 and resistance 32.
Apparatus for measuring voltage 36 measures voltage V1 that is produced between the terminals of resistance 32 by current i1, which has been introduced to resistance 32 via balun 20, and determines the value of current i1. Moreover, the voltage v1 between the terminals of component to be measured 18 is measured by apparatus for measuring voltage 38 via capacitor 34 and measurement V2 is obtained. Impedance Zx=v1/1 of component to be measured 18 is obtained by multiplying a predefined coefficient A by ratio V2/V1 of measurements V2 and V1. Power consumption in the component to be measured 18 is obtained by multiplying a predefined B by a product of V2 and V1. The ratio between current i2 to i1 that produces voltage V1 and i1 must be stabilized for stability of coefficients A and B after the calibration for measurements. The reason why this stability is lost is that there are changes in the values of the component to be measured as well as fluctuations in balun properties due to changes in temperature, etc.
The ratio between currents i1 and i2 in FIG. 1 is calculated by the following formula:
xe2x80x83i1/i2=xe2x88x92{R1+R2+Zc3)/R2}xc3x97N1/N2xe2x80x83xe2x80x83(Formula 1)
Here, N1={1+Zc1/(R1+R2+Zc3)+(Zc1/Z1)xc3x97(R3+Zc3)/(R1+R2+Zc3)}, N2={1xe2x88x92(Zc1/Z1)xc3x97(Zx/R2)} and R1, R2 and R3 are the resistance values of resistance""s 14, 16, and 32, respectively; Zc1 and Zc3 are the impedance values of capacitors 24 and 30, respectively, and Zx and Z1 are the impedance value of component to be measured 18 and the self-impedance value of balun 20, respectively.
The self-inductance of the above-mentioned transmission path-type coaxial balun is dependent on the magnetic permeability of the ferrite core and therefore, is unstable with changes in temperature. Therefore, an attempt will be made to investigate the effect of the value Z1 of self-impedance on current ratio i1/i2. The denominator in formula 1 becomes a function of impedance Zx of the object to be measured and therefore, the case where the impedance of the component to be measured is 500xcexa9 will be studied as an example. First, a capacitor and resistance are used, whose temperature coefficient of the component values less than 100 ppm/xc2x0C. can be easily obtained, and therefore, changes in the impedance of these components can be disregarded. However, the self-impedance of the balun is dependent on the magnetic permeability of the core that is used in this balun and therefore, is about 0.5%/xc2x0C. The absolute self-impedance value of the balun changes by 10% with a change in temperature of 20xc2x0 C.
When typical impedance values (R1=R2=R3=50xcexa9, Zc1=Zc3=xe2x88x92j0.5) xcexa9, Zx=500xcexa9, Z1=j100xcexa9; (here, j is an imaginary number) are substituted in above-mentioned (formula 2), it is clear that a change of 0.5% is produced in the value of i1/i2 with a change of 10% in self-inductance Z1 of the balun. This type of change can lead directly to errors in measurements of impedance.
While, it is clear that when Zc1=0 (that is, when C1 is reduced), N1=0 and N2 does not =0 then and changes in the value of i1/i2 are not produced with a change in self-impedance Z1 of the balun. However, direct current cannot be applied to the component to be measured with a structure wherein Zc1=0.
Although the case where 500xcexa9 is the impedance Zx of component to be measured 18 was studied here, the change in i1/i2 when 500xcexa9 is replaced by 50xcexa9 becomes approximately 0.1%. Thus, this amount of change in i1/i2 is greatly dependent on the value of the component to be determined and measurement errors will increase therefore so-called 3-point correction may not be correctly performed. Moreover, temperature correction is also dependent on the absolute self-impedance of the balun and is not realistic.
The object of the present invention is to provide a current detector with which alternating current can be detected with stability over a broad band, even if direct current has been added.
Another object of the present invention may be to present a high-precision instrument for measuring impedance that uses this current detector.
Yet another object of the present invention may be to present an apparatus for measuring power that uses this current detector.
The main structure of the present invention is given below:
A first embodiment of the present invention is a current detector, comprising a first terminal, which receives power source current; a second terminal, which feeds output current to an external device; a third terminal, which outputs monitor current having a predefined relationship with said output current; a fourth terminal having reference potential; a first component connected between the first and second terminals, a first balun, which comprises the first and second terminals as a first pair of input terminals and has a first pair of output terminals connected by a first line to the above-mentioned first pair of input terminals: a second balun, which comprises the above-mentioned first pair of output terminals as a second pair of input terminals and has a second pair of output terminals connected by a second line to the above-mentioned second pair of input terminals: a first capacitive component, which is connected between one output terminal of the above-mentioned first pair of output terminals having direct-current coupling with the above-mentioned first terminal and the above-mentioned fourth terminal; and a second capacitive component, which is connected between one of the output terminals of the above-mentioned second pair of output terminals having direct-current coupling with the above-mentioned first terminal and the above-mentioned fourth terminal, wherein the other output terminal of the above-mentioned second pair of output terminals having direct-current coupling with the above-mentioned second terminal serves as the third terminal.
The above-mentioned first component can be a resistance component.
At least one of the above-mentioned first and second baluns can be a balun wherein at least one circuit corresponding to the above-mentioned first and second circuits is coiled around a ferrite core.
Furthermore, at least one of the above-mentioned first and second lines can be a coaxial line.
By means of the present invention, only one coaxial line is used for both the above-mentioned first and second lines.
The above-mentioned first terminal may have a direct-current coupling with the outer conductor of the above-mentioned coaxial line.
In addition, the current detector further comprises an apparatus for measuring current connected to the third terminal, with which current is received from the above-mentioned third terminal and measurements corresponding to the above-mentioned pilot current are provided.
The above-mentioned apparatus for measuring current may have a third capacitive component having one terminal connected to the third terminal, an input resistance component connected between the other terminal of the above-mentioned third capacitive component and the fourth terminal, and an apparatus for measuring voltage, which is coupled with the above-mentioned input resistance component and is for measuring the voltage produced at the above-mentioned input resistance component.
In addition, the present invention gives an instrument for measuring impedance comprising the above-mentioned current detector, a voltage detector connected between the above-mentioned second and fourth terminals, which provides measurements in accordance with voltage produced between the above-mentioned second and fourth terminals, and a control and computation means, which calculates measurements related to the impedance to be measured between the above-mentioned second and fourth terminals from the measurement corresponding to the above-mentioned voltage and the measurement corresponding to the above-mentioned current.
The present invention provides an apparatus for measuring power comprising the above-mentioned current detector, a voltage detector connected between the above-mentioned second and fourth terminals, which provides measurements in accordance with the voltage produced between the above-mentioned second and fourth terminals, and a control and computation means, which calculates measurements related to the power consumed by the component to be measured between the above-mentioned second and fourth terminals from the measurement corresponding to the above-mentioned voltage and the measurement corresponding to the above-mentioned current.
The other details of the invention and the results of the same can be easily understood from the following description in the present Specification.